Ultrahigh-frequency attenuator



Filed Sept. 5, 1950 IN V EN TORS W WWW,

Jdmes Wfikaw" R. M. w. JOHNSON ET AL ULTRAHIGH-FREQUENCY ATTENUATOR Dec.2, 19 52 I l I A \N \N m N Patented Dec. 2, 1952 UNITED STATES PATENTOFFICE ULTRAHIGH-FREQUENCY ATTENUATOR Ralph M. W. Johnson and James W.Shaw, Los Angeles, Calif., assignors. to Stoddart Aircraft RadioCompany, Los Angcles, Calif.

Application September 5, 1950, Serial No. 183,202

Claims. .1.-

This invention relates to ultra high frequency apparatus, and toresistive attenuators incorporated therein and more particularly relatesto an improved T section resistive attenuator designed for use in anultra high frequency coaxial trans mission line.

The transmission of energy at ultra high frequencies requires apparatusspecially designed for that purpose if the energy to be passed throughthe transmission line is to suffer but slight attenuation and if thatinitial energy is to be maintained at a constant and unchanging phaserelationship. It, however, i recognized that this ideal situation doesnot ordinarily exist ina transmission line of any appreciable length andfurther that at such frequencies discontinuities within the line and thechanging electrical and physical characteristic of the coaxial lineitself may result in phase errors and undesired attenuation of thesignal to be transmitted.

It is therefore one purpose of this invention to provide a.device whichwill compensate for phase errors and undesired attenuation existing in agiven transmission line.

It is well. recognized in the art dealing with It is apparent to thoseskilled in this art that a transmission line has a given characteristicimpedance depending upon. the physical and electrical parameters of theline and that this characteristic impedance may change over a givenlengthof transmissionline. These special networks may correct for thischanging characteristic impedance of a transmission line in order tomake it appear more resistive over the frequency range to betransmitted. This function of such special networks is of'particular.importance in those situations in which the coaxial transmission line isto serve ina circuit designed for duplex operation, 1. e., thetransmission line is designed to handle currents traveling in eitherdirection throughout itslength.

In any such transmission system it is desirable to have impedancematching between elements of the line so that there will be a minimum ofdiscontinuity in the system. While it is reco nized that a particularresistive configuration maybe employed in an ultra high frequencytransmission line to correct for the changing characteristic impedanceof the line, an ultra high frequency attenuator designed to accomplishthe above mentioned functions must be made so as to avoid any impedancemis-match between the transmission line and the attenuator itself. Itwill be recognized that under a desirable matching condition all energyto be transmitted in one direction is so transmitted; while in theundesirable condition of a mis-match between the coaxial transmissionline and the ultra high frequency attenuator, energy which is to betransmitted in one direction is in part reflected back into thetransmission line thus affecting the standing Wave ratio (SWR) which inan ideal system should approximate unity. Accordingly it is one of theobjects of this invention to provide new and improved resistiveattenuators and more particularly a coaxial T section resistiveattenuator which results in a standing wave ratio of approximately unitywhen suitably positioned in a concentric transmission line.

It is another object of this invention to provide an ultra frequencyattenuator which may be suitably positioned in a transmission line so asto correct for changing characteristics impedance of the transmissionline itself.

It is still another object of this invention to provide such anattenuator which may be positionedin a transmission line so constructedand designed as to avoid an impedance mis-m-a'tch between thetransmission line and the attenuator itself. It'is a further object ofthe invention to provide a new and improved attenuator comprising a discresistor positioned between two coaxial sections, the outer section ofeach having a tapered inner surface and coaxially disposed therewithinner tubular resistive sections.

It is a further object of this invention to provide a new andnovelattenuator which is symmetrical in nature and when suitably positionedin a transmission line will render the impedance at its point ofinsertion substantially resistive in both directions.

It is a further object of this invention to-provide such an attenuatorwhich may be readilyand easily manufactured, assembled, and incorporatedin a coaxial transmission line.

Forv a more complete understanding of this invention, reference maybemade to the following description taken in conjunction with theaccompanying drawing. The'scope of this invention 3 will further bepointed out in the appended claims.

Fig. l is a longitudinal cross sectional view of one embodiment of thisinvention and Fig. 2 is a longitudinally cross sectional View of theassembled coaxial 1' section resistive attenuator.

This latter figure illustrates not only the novel attenuator but alsoconventional coupling means whereby the attenuator may be readilyinserted in a concentric line.

In order to obtain a 10W standing wave ratio in the coaxial T attenuatoritself, cognizance must first be taken to the variation in theresistance of the center conductor which in the subject attenuator formsthe resistive series element. Although this series element may takevarious forms it is herein described as the hollow cylindrical member 6.This series arm may be thought of as a conventional filmresistor in thatit may be a tubular member coated on its outer surface with carbongraphite or like conductive material. For a further and more completedescription of such a resistor reference is made to the copendingapplication Serial No. 155,422.

The impedance of this coaxial configuration, although it may changethroughout its length, should be essentially resistive in nature andinasmuch as the resistance of the inner tubular film resistors varyalong its length in much the same manner as the resistance of the innerconductor of a coaxial transmission line may vary from Z to a lowervalue, the impedance of the line and of the attenuator itself must alsobe made to vary simultaneously. To avoid any mis-match between atransmission line and the attenuator, the inner surface of the outerconductor tapers inwardlyfrom a comparatively large diameter adjacentthe coupling to the transmission line to a point adjacentthe innertubular resistor and the disc resistor which forms the shunt portion ofthe attenuator.

The disc resistor 8 is conventional in nature and form a resistance pathradially extending from the inner conductor or resistor 6 to the outerportion where it makes contact at the outer edge with the speciallytapered outer conductor 3. In order to present minimum of discontinuityto radio frequency currents traveling through the disc, it is desirableto design the characteristic impedance of the body of the disc to matchthe resistance of the film or films on the disc. This may be done bycontrol of the dielectric constant of the disc as well as by choice ofdimensions of the inner and outer diameters. For a more completedescription of a particular type of disc resistor reference is made tothe copending application Serial No. 155,422, filed April 12, 1950. a

Referring to Fig. 1, there is illustrated an ultra high frequencyattenuator I having an outer conductor 3 whose inner surface 2 tapersinwardly to a point 4 adjacent to but spaced apart from the outersurface 5 of the hollow cylindrical film resistor B. The inner surface 2of the outer conductor 3 forms a throat portion at 4 which abruptlyflares outwardly, forming tapered wall surface 14 from that throatportion towards one terminal portion 1 of the film resistor 6 and spacedapart from the disc resistor 8, contacting the disc resistor at theouter edge 9 of the inner surface 10. The film resistor 6 incorporates,in addition to a conducting coating ll, metallic terminal receiving endportions 1 and [2, the end portion 1 remote from the large diametersurface of the outer conductor fitting snugly into a centrally disposedaperture I3 of the disc resistor.

The inner tapered surface 2 of the outer conductor 3 performs theessential functions of matching the transmission line to the resistiveattenuator l and prevents a mis-match within the attenuator itself. Itis to be again emphasized that this tapered portion, surface 2, forms athroat at 4 which abruptly flares outwardly to the outer surface of theconductor I. This abrupt flared surface I4 is an important feature inthat not only does it prevent the short circuiting of the currentsfiowing in the disc resistor 8 but also prevents any discontinuity whichmight arise from an abrupt non-tapered configuration.

Fig. 2 illustrates a T section resistive attenuator having conventionalcoaxial coupling means 15 and I6 so that the attenuator may be suitablypositioned in a transmission line. While the details of these couplingmeans are not shown, inasmuch as they are common in the art, it might bementioned that they essentially comprise an outer conductor portionadapted to mate with the outer conductor of the connecting coaxial cableand an inner centrally disposed center conductor forming the remote endsof socket portions adapted to receive a plug connector from the coaxialtransmission line. The terminal connecting means are illustrated only inoutline since the specific construction forms no part of this invention.

The resistive attenuator I1 is encased in a hollow cylindrical housingl8 which housing imparts rigidity to the structure.

An important feature to notice in this embodiment is the split retainersleeve or shim [9. This.

member compensates for manufacturing tolerances and accurately providesmeans whereby the attenuator may be accurately positioned within thetubular housing l8.

One element of the attenuator comprises the centrally positioned discresistor 8 whose annular outer surface 20 contacts thefinner surface ofthe retaining sleeve [9 and which incorporates a centrally disposedaperture 7 within which suitable terminal receiving means arepositioned. The terminal receiving means are of particular importance inthat they provide a positive support for the hollow tubular resistors 2|and 22 heretofore described in connection with Figure 2 and providepositive electrical contact with the end portions of said inner tubularresistors. The terminal receiving means comprises a longitudinally splitsleeve member 23 positioned Within the centrally disposed aperture 7 ofthedisc resistor 8 and extending outwardly from both plane surfaces 24and 25 of the disc resistor. A split retaining ring 26, which isoutwardly biased, is centrally positioned within the terminal sleeve andserves to hold the center terminal receiving means accurately andsecurely within the disc resistor.

The attenuator in addition comprises two outer conductor portions 21 and28 similar in structure to tapered member 3 described in connection withFig. 1 positioned on either side of the disc resistor and havingsurfaces which taper inwardly from the end portions l5 and 16 adjacentthe coaxial coupling means to a point adjacent to but spaced apart fromthe centrally located disc resistor B and forming throat portions 29 and30 at that point.

Centrally disposed within the hollow cylindrical outer conductor are.two Series resistive elements,

2| and 22 similar in structure to the hollow cylindrical member 6described in connection with Fig. 1, whose ends adjacent the coaxialcoupling means make electrical contact with the central conductor 3| ofthe coaxial line by means of a centrally positioned terminal meansconnected thereto and whose other end portions are received by theterminal receiving means located within the centrally positioned discresistor. Metallic terminals 33 of the tubular resistors fit into thesleeve terminal and retaining member or socket 32 which is supportedwithin the outer tubular housing or casing by insulating beads 34 havinga suitably disposed centrally located aperture. The sleeve plug makeselectrical connection with a metallic inner conductor 3| forming aportion of the central conductor of the coaxial transmission line wheninserted in such a line.

Referring again to the outer conductors 28 and 21 and their relationshipto the coaxially disposed hollow cylindrical resistive inner conductors,it is again emphasized that the inner surface which is inwardly taperedtowards the centrally disposed disc resistor does not make electric-a1contact with the outer surface 5 of the resistive elements nor with thecentral portion of the disc resistor. The two ends of these outerconductors remote from the coaxial cable coupling means form wallsurfaces 35 and 36 which taper abruptly outwardly from the throatportions 29 and 30 to the outer surface of the outer conductor, at whichpoint they make positive electrical contact with the outer edges of theplane surfaces 24 and 25 of the disc resistor 8.

While particular embodiments of our invention have been shown, we do notwish to be limited thereto since other modifications may be made andtherefore the appended claims are intended to cover any suchmodifications as may fall within the true spirit and scope of thisinvention.

What We claim as novel and desire to secure by Letters Patent of theUnited States is:

1. An ultra high frequency attenuator for insertion in a coaxialtransmission line comprising an outer hollow cylindrical conductorhaving an inner surface which tapers inwardly from one end thereof to apoint adjacent the other end and forming a throat portion at said point,said inner surface forming an abruptly flared outwardly extending wallportion from said point to the other end of said outer conductor, a discresistor having an inner and outer plane surface which inner surfacecontacts peripherally said abruptly flared outwardly extending wallportion of said outer conductor adjacent said throat portion and havinga centrally disposed aperture, and a tubular film resistor coaxiallypositioned within said outer conductor and spaced apart therefrom makingelectrical contact at one end thereof and positioned within saidaperture.

2. In an ultra high frequency attenuator for use in electricallyconnecting and matching sections of transmission lines having an innerconductor and a spaced apart outer conductor, the combination of anouter hollow cylindrical conductor, an inner tubular resistor coaxiallydisposed therein, and a disc resistor having a centrally disposedterminal receiving means, said outer conductor having an inner surfacewhich tapers inwardly from one end thereof to a point adjacent the otherend and forming a throat portion at said point, said inner surfaceforming an abruptly flared outwardly extending wall portion from saidpoint to the other end of said outer conductor, said disc resistorhaving a plane surface contacting peripherally of said abruptly flaredoutwardly extending wall portion of said outer conductor adjacent saidthroat portion, said tubular resistor. being spaced apart from saidouter conductor and supported at one end thereof by and makingelectrical contact with said terminal receiving means.

3. In an ultra high frequency attenuator for use in electricallyconnecting and matching sections of a transmission li'n'eliaving aninner conductor and aspa'ced'apart. outer conductor, the combinationcomprising a hollow cylindrical casing, a centrally positioned discresistor within said casing, outer hollow cylndrical conductors withinsaid casing and positioned on both sides of said disc resistor,cylindrical resistors coaxially disposed within and spaced apart fromsaid outer conductors, each of said outer conductors having an innersurface which tapers inwardly toward said disc resistor to a pointadjacent said disc resistor and forming a throat portion at said point,said inner surfac forming an abruptly flared outwardly extending wallportion from said point towards said disc resistor and making electricalcontact peripherally therewith, each of said cylindrical resistors beingsupported at one end thereof by and making electrical contact withterminal receiving means centrally positioned within said disc resistor.

4. An ultra high frequency attenuator for use in a coaxial transmissionline and having coaxial coupling means at remote ends thereof.comprising hollow cylindrical casing, a longitudinally split cylindricalsleeve coaxially disposed and co-extensive with said casing, a centrallypositioned disc resistor within said sleeve and having axiallypositioned terminal receiving means a pair of outer hollow cylindricalconductor members within said sleeve, each of said members beingdisposed on opposite sides of said disc resistor, and each of saidmembers having an inner surface which tapers inwardly towards said discresistor to a point adjacent said disc resistor forming a throat portionat said point, said inner surface forming an abruptly flared outwardlyextending wall portion from said point towards said disc resistor andmaking electrical contact peripherally therewith, a pair of tubularresistors each of which are coaxially disposed within and spaced apartfrom its surrounding outer conductor and being supported and makingelectrical contact at one end by said coaxial coupling means and at theother by said terminal receiving means.

5. In an ultra high frequency attenuator coupling device having coaxialcoupling means at remote ends thereof for use in electrically connectingand matching sections of transmission line, the combination comprising ahollow cylindrical casing, a longitudinally split cylindrical sleevecoaxially disposed within said casing, a centrally positioned discresistor within said sleeve and having a centrally located aperture,terminal receiving means located within said aperture comprising aninner split sleeve portion secured within said aperture by an outwardlybiased split retaining ring, a pair of outer hollow cylindricalconductor members positioned within said sleeve, each of said membersbeing positioned on opposite sides of said disc resistor and each havingan inner surface which tapers inwardly towards said disc resistor to apoint adjacent said disc resistor forming a throat portion at saidpoint, said inner surface forming an abruptly flared outwardly extendingwall portion from said point towards said disc resistor and making 7electrical contact peripherally therewith, a pair REFERENCES CITED oftubular resistors each of which is coaxianyv The following referencesare of record in the disposed within and spaced apart from one of saidouter conductors and supported and making me of thls patent electricalcontact at one end thereof by said co-' UNITED STATES PATENTS I axialcoupling means and at the other end by' Number Name Date sa1d centrallylocated termlnal receiving means. 1 957,538 Jensen 8 1934 RALPH M W.JOHNSON 2,4 8,775 Ovreb0 May 3, 1949 JAMES W. SHAW. 10 2,521,828Chatterton et al. Sept. 1950

